HEVs (Hybrid Electric Vehicles) typically include an ISA (Integrated Starter and Alternator), which is an apparatus that functions as a starter, alternator, and torque assister. When the ISA is provided as an induction motor the amount of back electromotive force (back EMF) and reactive power generated by the ISA is relatively small, provided its angular velocity is low. As the angular velocity of the ISA increases, however, the amount of back EMF and reactive power also increases. If the back EMF exceeds the maximum voltage that can be supplied from a D.C. power source such as a battery, it becomes difficult to control the angular velocity of the ISA and the torque of the induction motor.
In the control of an induction motor, back EMF compensation and reactive power compensation can be performed by a single power inverter system, such that output power converted by the inverter system is decreased in proportion to an increase in reactive power. To prevent the output power from decreasing, the back EMF constant can be decreased. When the back EMF constant is decreased, however, the torque constant is also decreased resulting in the deterioration of regeneration power.
A reduction gear can be adapted to compensate the decreased output torque of the induction motor, but when the reduction gear is adapted, energy loss during power transfer increases such that the overall energy efficiency of the system decreases.
Accordingly, what is needed is a vector controlled, dual inverter system and method capable of controlling the angular velocity of an ISA and the torque of an induction motor without decreasing the overall energy efficiency of the system.